The present invention relates to a new and improved construction of an infrared intrusion detector which is of the type containing a pyroelectric detector element and an evaluation circuit connected therewith for giving an alarm signal in the presence of a change in radiation of the detector element caused by a moving object, typically an individual.
In the case of infrared intrusion detectors, such as for instance disclosed in U.S. Pat. No. 3,703,718, granted Nov. 21, 1972 or United States Pat. No. 4,058,726, granted Nov. 15, 1977, the infrared radiation emitted by a human at receiving regions or fields of view is received by a suitable detector element or detector. An evaluation circuit connected with this detector element, such as for instance disclosed in the aforementioned U.S. Pat. No. 3,703,718 or U.S. Pat. No. 4,179,691, granted Dec. 18, 1979, delivers an alarm signal as soon as the infrared radiation received by the detector element alters in a manner characteristic for the movement of a person at the monitored region.
Previously there was used as the detector element in such infrared intrusion detectors usually sensors which altered their electrical resistance when they were irradiated, or those which when impinged by infrared radiation delivered a voltage at their output, for instance bolometers, photoresistors or photoelements. The evaluation circuits connected therewith were provided at their input usually with a suitable amplifier element which, when working with high-ohm sensors, normally was designed as a field-effect transistor. In order to be able to detect changes in the radiation impingement a differentiation element usually was provided before or after the imput amplifier.
In more recent times pyroelectric detector elements have been found to be extremely suitable for use with infrared intrusion detectors. These pyroelectric detector elements also can be manufactured in a foil configuration and then can be particularly flexibly or universally adapted as concerns their dimensions and shape to the contemplated field of use and in the case of infrared intrusion detectors particularly to the desired receiving regions or fields of view.
Heretofore known pyroelectric detectors for instance those formed of single crystals, such as for example lithium tantalate, LiTaO.sub.3, ceramics, for instance lead-zirconate-titanate, PZT, or lead-zirconate-iron-niobate or foils of polyvinyldifluoride (PVF.sub.2) consist of a ferroelectric material, i.e. of a dielectric having a certain capacitance which is governed by the dimensions and thickness of the detector. When receiving infrared radiation the dielectric heats-up and there is formed an electrical potential by virtue of the generated charge. This electrical potential is dependent upon the pyroelectric coefficient of the material which indicates the amount of electrical charge which has been generated per unit area and per temperature change as well as upon the capacitance of the detector element, i.e. upon the dielectric constant of the material.
During the evaluation of such output voltage of the pyroelectric detector it is necessary to use an extremely high-ohm voltage amplifier having an input resistance of at least 10.sup.10 ohms because of the high internal resistance. Thus, as has been disclosed in British Pat. No. 2,021,761 published Dec. 5, 1979 or U.S. Pat. No. 3,839,640, granted Oct. 1, 1974, there is usually used for this purpose a field-effect transistor, the gate electrode of which is directly controlled by the pyroelectric detector. What is disadvantageous with this circuit arrangement is that because of the high-ohm removal of the voltage there occurs a large resistance or thermal noise at the amplifier input and the connection of the detector with the amplifier is extremely sensitive to stray capacitances and electrical disturbances.
Special screening measures and a special assembly of the amplifier element are therefore necessary. Additionally, the time-constant of the detector is relatively large and is in the order of a number of seconds, so that the value of the time-constants additionally is dependant upon the detector capacitance, i.e. upon the dimensions of the detector element, which in each instance requires a special accommodation or adaptation.
A further drawback of such evaluation circuits working with a high-ohm voltage amplifier resides in the fact that the signal voltage generated by a change in radiation also is dependant upon the capacitance of the detector element, in other words upon the dielectric constant of the material. By virtue of the foregoing during the fabrication of such detectors there is decisive, apart from thermal properties which can be influenced by the construction, not solely the pyroelectric coefficient, but also the dielectric constant. The particularly great pyroelectric coefficient of certain materials, for instance that of PZT therefore cannot be utilized, since at the same time the dielectric constant is extremely high and such detectors therefore deliver a smaller signal when using known evaluation circuits than detectors having poorer pyroelectric properties and smaller dielectric constants.